The present invention relates to a transistor circuit employing insulated gate field effect transistors, and more particularly to a voltage-current converter circuit.
Insulated gate field effect transistors (hereinafter abbreviated as IGFET's) have been widely used. Among their principal application, there exists a current source for supplying or absorbing a predetermined value of current. Such current sources are utilized, for example, as a constant current source for a differential amplifier or a current source for effecting charge or discharge of a time constant circuit. In accordance with improvements in circuit techniques in recent years, a capability of controlling a current value has been desired for a current source. In addition, due to the fact that power supply voltages have become low-voltage, stabilization of an operation at a low voltage and operations over a wide voltage range have been required.
As is well-known, a source-drain current of an IGFET would not vary linearly as a fuction of a gate-source voltage. More particularly, representing a gate-source voltage by V.sub.GS, a drain-source current by I.sub.DS, a threshold voltage by V.sub.T and a current amplification factor by .beta., when the drain-source voltage V.sub.DS fulfils the condition of V.sub.DS &gt;V.sub.GS -V.sub.T, the following relation is established: EQU I.sub.DS =.beta.(V.sub.GS -V.sub.T).sup.2 ( 1)
and therefore, the drain-source current I.sub.DS has a square (the second powered) characteristic with respect to the gate-source voltage V.sub.GS. In various applications, this becomes great obstruction in the case where an IGFET is used in a linear circuit for which a linear relation beween a voltage and a current is required.
However, even with IGFET's having such a characteristic, it is possible to contrive to obtain a linear relation between a voltage and a current. Now it is assumed that two IGFET's having respective threshold voltages V.sub.T1 and V.sub.T2 and an identical current amplification factor .beta. are prepared and a common voltage V.sub.GS is applied between their gates and sources. Then, the respective drain-source currents I.sub.DS1 and I.sub.DS2 are represented by the following equations, similarly to Equation-(1) above: EQU I.sub.DS1 =.beta.(V.sub.GS =V.sub.T1).sup.2 ( 2) EQU I.sub.DS2 =.beta.(V.sub.GS -V.sub.T2).sup.2 ( 3)
At this moment, the respective drain-source voltages V.sub.DS1 and V.sub.DS2 fulfil the relations of V.sub.GS1 &gt;V.sub.GS -V.sub.T1 and V.sub.DS2 &gt;V.sub.GS -V.sub.T2. Considering now the difference between these currents flowing through the respective IGFET's, from Equations-(2) and -(3) above it can be seen that the following relation is established. EQU I.sub.DS1 -I.sub.DS2 =2.beta.V.sub.GS (V.sub.T2 -V.sub.T1)+V.sub.T1.sup.2 -V.sub.T2.sup.2 ( 4)
In other words, the difference current between the drain-source currents of the two IGFET's has a linear relationship to the common gate-source voltage V.sub.GS. Accordingly, if provision is made such that when the same gate-source voltage is applied to two IGFET's having different threshold voltages and the same current amplification factor, the difference current between the drain-source currents of the respective IGFET's can be detected. Thus, even in a circuit constructed of IGFET's it is possible to realize a linear voltage-current characteristic.
Although a sum of currents flowing through two IGFET's, respectively, can be obtained simply by connecting the IGFET's in parallel, a simple method for obtaining a difference between two currents has not been known.